Workaround brain-damaged conversions

include/fmt/core.h

@@ -286,8 +286,10 @@ struct monostate {
 #endif
 
 // This is defined in core.h instead of format.h to avoid injecting in std.
+// It is a template to avoid undesirable implicit conversions to std::byte.
 #ifdef __cpp_lib_byte
-inline auto format_as(std::byte b) -> unsigned char {
+template <typename T, FMT_ENABLE_IF(std::is_same<T, std::byte>::value)>
+inline auto format_as(T b) -> unsigned char {
   return static_cast<unsigned char>(b);
 }
 #endif

test/core-test.cc

@@ -831,3 +831,28 @@ TEST(core_test, has_const_formatter) {
 TEST(core_test, format_nonconst) {
   EXPECT_EQ(fmt::format("{}", nonconst_formattable()), "test");
 }
+
+struct its_a_trap {
+  template <typename T> operator T() const {
+    auto v = T();
+    v.x = 42;
+    return v;
+  }
+};
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<its_a_trap> {
+  auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  auto format(its_a_trap, format_context& ctx) const -> decltype(ctx.out()) {
+    auto s = string_view("42");
+    return std::copy(s.begin(), s.end(), ctx.out());
+  }
+};
+FMT_END_NAMESPACE
+
+TEST(core_test, trappy_conversion) {
+  EXPECT_EQ(fmt::format("{}", its_a_trap()), "42");
+}